Problem: You have found the following ages (in years) of 4 zebras. Those zebras were randomly selected from the 36 zebras at your local zoo: $ 3,\enspace 23,\enspace 8,\enspace 5$ Based on your sample, what is the average age of the zebras? What is the standard deviation? You may round your answers to the nearest tenth.
Answer: Because we only have data for a small sample of the 36 zebras, we are only able to estimate the population mean and standard deviation by finding the sample mean $({\overline{x}})$ and sample standard deviation $({s})$ To find the sample mean , add up the values of all $4$ samples and divide by $4$ $ {\overline{x}} = \dfrac{\sum\limits_{i=1}^{{n}} x_i}{{n}} = \dfrac{\sum\limits_{i=1}^{{4}} x_i}{{4}} $ $ {\overline{x}} = \dfrac{3 + 23 + 8 + 5}{{4}} = {9.8\text{ years old}} $ Find the squared deviations from the mean for each sample. Since we don't know the population mean, estimate the mean by using the sample mean we just calculated {46.24} + {174.24} + {3.24} + {23.04}} {{4 - 1}} $ {s^2} = \dfrac{{246.76}}{{3}} = {82.25\text{ years}^2} $ As you might guess from the notation, the sample standard deviation $({s})$ is found by taking the square root of the sample variance $({s^2})$ ${s} = \sqrt{{s^2}}$ $ {s} = \sqrt{{82.25\text{ years}^2}} = {9.1\text{ years}} $ We can estimate that the average zebra at the zoo is 9.8 years old. There is also a standard deviation of 9.1 years.